How To Build a Pinewood Derby Car/Assembly
Wheel Alignment edit
Wheel alignment is the process of aligning the wheel vertically (camber) and parallel to the direction of travel (toe and steer angles). When properly aligned, the wheel will "float" in the middle of the axle when the car is moving. This will reduce friction at the contact points between the wheel hub and the body or axle head.
Before starting the alignment procedure, mark each axle head to identify it as the up position. Always insert the axle with the mark facing up in the 12 o'clock position. This will assure that any effects of a slight bend in the axle can be removed by shimming. Each axle and wheel should be aligned and run at the same position. Use a muffin tin or egg carton bottom to store the wheels and axles during alignment. Mark the container Left/Right Front/Rear and put any components that you aren't using for that alignment step in the appropriate place.
Rough alignment edit
The goal of coarse alignment is to create an axle insertion hole in which the axle can be inserted perpendicular to the vertical and horizontal axis of the car.
Insert a single axle (without the wheel) in the car body. Do not use a hammer, instead place the axle partially into the hole and gently press it into the body. Grasp the car body firmly and press the axle head against a flat surface and insert the axle until about 7/16" is protruding. The surface should not be hard or slippery; a wooden workbench is ideal. If you are using axle slots, an axle insertion tool can be useful.
The axle should have a snug press fit in the axle hole. If the axle is too tight, don't force it. A small amount of wax can be used to for a particularly stubborn axle. A #44 drill bit (0.086 inch decimal equivalent) held between the thumb and forefinger can be used to gently ream the axle hole and remove any paint, glue or body putty that may have accumulated there.
Check the angle between the axle and body using a small ruler. When the ruler is flat against the car and touching the axle, there should be no gap at the inner or outer part of the axle. If there is, shim or bend the axle (see below) until it is straight.
Fine alignment edit
Fine alignment is accomplished by rolling the car back and forth on a flat, smooth surface and observing the behavior of the wheels. Observe and make changes to each wheel individually.
Camber: If the wheel moves to the axle nail head when the car is moved backward and forward, then that wheel has a negative camber; the axle head is angled upward and should be adjusted downward by shimming on the top of the axle. If the wheel moves toward the body when the car is moved backward and forward, then that wheel has a positive camber; the axle head is angled downward and should be adjusted upward by shimming on the bottom of the axle.
Toe/Steer: If the wheel moves to the body when the car is moved forward and to the axle head when the car is moved backward, then that wheel is toed in; the axle head is angled forward and should be adjusted rearward by shimming on the front side of the axle. If the wheel moves to the axle head when the car is moved forward and to the body when the car is moved backward, then that wheel is toed out; the axle head is angled backward and should be adjusted forward by shimming on the rear side of the axle.
Alignment should be performed on a smooth, flat surface with a slight incline. The rise of the incline should be no more than 1/4" inch over the course of several feet (half a degree or less of angle). The car will roll slowly or can be pushed gently down the slope. Place a stop at the top and bottom of the slope.
Alignment is accomplished with multiple trials of running the car down the slope with the front end facing down the slope or, in reverse, with the rear facing down the slope in the direction of travel. Allow the car to run about a foot down the slope and observe and record your observations for one of the wheels.
Start with the front wheel; in the raised wheel configuration, concentrate on the wheel that is in contact with the track. First assure that the car moves straight down the track without turning in one direction or the other. Shim the axle (see below) to turn the wheel with respect to the car body so that the car tracks straight.
Once the car tracks straight, observe the "IN-OUT" behavior of the wheels. Here, IN is toward the body and OUT is away from the body. Perform two trials with the car facing forward. First, move the wheel in to touch the body and roll (or let the car roll) down the slope. Next, move the wheel to the end of the axle and repeat the forward trial. If the wheel moves from the inside to the outside, and stays at the outside when starting there, the result of this trial is OUT. Likewise, if the wheel moves from the outside to the inside, and stays at the inside when starting there, the result of the trial is IN. If the wheel stays put each time, the axle is aligned or nearly aligned.
Reverse the car so that the rear is facing down the slope and repeat the trial to determine IN, OUT, or no movement.
If the wheel always moves to the inside (IN-IN), this means that the axles are canted downward and the downward force of the body tends to move the wheels in no matter which way the car is rolling. Shim or bend the axle slightly upward and repeat the trials.
If the wheel always moves to the outside (OUT-OUT) it is canted upward and must be shimmed or bent downward.
If the wheel moves out when the car is facing forward and in when facing backward (OUT-IN), this is an indication of rearward cant. Shim or bend the axle forward.
If the wheel moves in facing forward and out facing backward (IN-OUT), the axle is canted forward and should be shimmed or bent to a more rearward facing angle.
The axle angle should be changed slowly with repeated trials after small adjustments. Concentrate on one wheel at a time and use a pencil and notepad to record the results of the trials.
Wax paper shimming edit
The wax paper shimming method relies on small strips of wax paper inserted into the axle holes to change the angle of the axle. Cut a few dozen strips of wax paper 1/8" by 1/2". Bend the shim slightly along the long axis and insert it into the axle hole 1/8" and bend it back along the car body. Place the shim on the side of the axle opposite to the direction you want the axle to move. For example, if the axle is canted downward and you want to angle it back up, place the shim(s) at the bottom of the axle (6 o'clock position). If the axle is canted forward, shim forward, if it is canted backward, shim back. If the axle is tilted upward, place the shim at the 12 o'clock position.
Place one shim at a time and work on one wheel at a time while checking the IN/OUT behavior of the wheel. Keep track of how many shims are placed and their location (12 , 3, 6, 9 o'clock positions) in each axle hole. If a set of shims falls out or is damaged, simply replace them with the same number of new shims and the axle alignment should be comparable.
Once the alignment is compete, remove the excess wax paper by trimming with a sharp utility knife. Use care to avoid scratching the axles and be sure that there are no wax paper scraps inside the hub.
Wax paper is approximately one one-thousandth of an inch thick (25 µm). Neglecting compression, a single thickness of shim should change the axle angle 0.1 degrees.
Axle Lubrication edit
Lubricants are used on the wheel-axle and wheel-body contact areas.
Dry lubricants edit
- Powdered graphite is one of the most common dry lubricants for Derby racing. Alkyd-based graphite lubricating paint such as "EZ-Slide" can also be used.
- Molybdenum disulfide
- Molybdenum disulfide is a dry lubricant similar to graphite. Commercially it is known under the names "Moly Lube" and "Dri-slide" and is used for lubrication for guns, automobiles and bicycles. It is also an excellent lubricant for pinewood derby applications. Moly/graphite mixtures are also used.
- Teflon is the trade name for polytetrafluoroethylene (PTFE) polymer. Powdered Teflon is a common Derby car lubricant.
Liquid lubricants edit
- Krytox oils and greases are perfluoropolyether (PFPE) synthetic lubricants made by DuPont. Krytox 100 is an ISO Grade 5 oil with a coefficient of friction of 0.08 that has been used in pinewood derby racing (MSDS).
- Nyoil II is a general purpose lightweight synthetic hydrocarbon oil that has been used with pinewood derby racers (MSDS).
- Silicone is liquid lubricant that has been used with Derby racers. Not to be confused with the element silicon, silicones are polysiloxanes, an organic/inorganic polymer. Note that the carrier solvent in some spray silicon lubricants can melt the wheel plastic.
Spray on lubricants edit
Alkyd based graphite spray paint in naptha and mineral spirits carrier (MSDS) can be used to paint axles and portions of the car body with a thin graphite coating. Care should be taken with this spray since the naptha may dissolve plastic and paint. When the carrier solvent evaporates, the alkyd resin hardens and holds the graphite in place. Graphite coating is used on lawn mowers and farm equipment to keep cuttings and other material from sticking.
A simple bushing can be made from graphite tape: sticky on one side and graphite on the other. The bushing is stuck to the car body before the axle is inserted. When it is in place, the inner axle hub will rub against the bushing rather than the car body should the wheel move inward. A similar effect can be achieved by painting the portion of the body near the axle with graphite spray paint.